Monitoring devices comprising electromagnetic wave (EM-wave) sensors are particularly useful in surveillance applications. For example, intrusion detectors comprising PIR (passive infra-red) sensors are commonly deployed in homes, museums, banks, offices, and other commercial and/or industrial establishments for security surveillance. Such intrusion detectors usually operate to detect infra-red (IR) radiation radiated by a moving object and will trigger an alarm when the detected IR level exceeding a pre-determined threshold, corresponding to the detection of an approaching or intruding human being. It is known that living objects emit IR radiation at a wavelength of about 10-12 μm. The more sophisticated IR detectors are equipped with processing circuitry to distinguish between an approaching human body or other living bodies.
Intrusion monitoring devices are frequently subject to tampering by intruders. For example, intruders have been known to apply a mask of lacquer, which is opaque to IR radiation, onto the detection window of an intrusion window to cheat it. Hence, it will be beneficial if there can be provided improved EM-wave detectors, especially IR monitoring devices alleviating shortcomings of conventional devices.